TUESDAY, Sept. 9, 2014 (HealthDay News) -- Teens who use marijuana on a daily basis before they reach the age of 17 are more likely to have certain long-term problems than their non-using peers, new research from Australia suggests.

Compared to non-using teens, the study reports that adolescents who use pot, also known as cannabis, on a daily basis before age 17 are:

60 percent less likely to get a high school diploma or university degree;

at an eight times greater risk for using a range of other illegal drugs in their 20s;

and 18 times more likely to develop a long-term dependence on marijuana that goes beyond the age of 17.

"What we found is that although the effects were greatest for the daily users, there were also notable effects at the lower frequencies of cannabis use as well," said study lead author Edmund Silins. He is a research fellow with the faculty of medicine at the University of New South Wales' National Drug and Alcohol Research Centre in Sydney.

"In fact," he added, "the less-than-monthly use before the age of 17 was also associated with a degree of risk of adverse outcomes, although it was less, of course, than it was for daily users."

Silins and co-author Richard Mattick discussed their team's findings on Tuesday at a teleconference from Sydney. Their study is published in the September issue of The Lancet Psychiatry.

The authors pointed out that marijuana is currently the most commonly used illegal drug worldwide, with daily (or almost daily) use now seen among approximately 7 percent of high school seniors in the United States.

To explore the potential long-term consequences of marijuana use during adolescence, the study team analyzed the collective findings of three long-term investigations involving almost 3,800 men and women in Australia and New Zealand.

The specific focus was on the frequency of pot use during mid-adolescence until age 17, with exposure ranked on a scale ranging from never to daily use.

In addition, long-term developmental performance was tracked on seven different measures, including: graduating from high school; obtaining a university degree by the age of 25; development of marijuana dependence; depression; frequency of suicide attempts; use of other illegal drugs (including cocaine, heroin, amphetamines, hallucinogens, and/or prescription medications); and whether or not participants were financially dependent on government assistance between the ages of 27 and 30.

The result: even after accounting for a wide range of factors that could affect developmental performance (including age, gender, ethnicity, financial means and mental illness), the research team concluded that there was a "clear and consistent" link between the daily use of marijuana during adolescence and considerably worse long-term prospects.

The investigators found that poorer outcomes were reflected on every measure except for two: the likelihood for experiencing depression and for being dependent on welfare.

The authors further noted that the risk for poor long-term performance appeared to be "dose-dependent," with greater use translating into worse outcomes.

At the teleconference, Silins said he believes the findings "are generalizable to regions with similar rates of cannabis use and similar legislative regulations." The prevalence of pot use, he noted, is now 10 percent in Australia and 15 percent in New Zealand, compared with 16 percent in the United States, 12 percent in Canada and 6.5 percent in the United Kingdom.

But findings notwithstanding, co-author Mattick, also of the National Drug and Alcohol Research Centre in Sydney, stressed that the study authors were "not trying to advise regulators or legislators in a direct sense. What we are trying to do is make them aware -- given the controversy about cannabis use and its impact on well-being -- of these outcomes."

The researchers suggest that marijuana legalization efforts be "carefully assessed" to protect teens from the long-term problems that might arise as a result of youthful usage.

Paul Armentano, deputy director of the marijuana legalization advocacy group NORML, in New York City, said that "the presumption that criminalizing cannabis adequately prevents or limits young people from gaining access to cannabis is demonstrably false."

He said that "there is little debate among experts that the use of cannabis by young people, in particular daily use of cannabis, ought to be discouraged, just as young people's use of other potentially problematic intoxicants, such as alcohol and tobacco, ought to similarly be discouraged."

But, Armentano added, "criminalizing cannabis for adults has little if any impact on reducing teens' access or consumption of the plant."

He suggested that a public policy that "regulates adult marijuana use, but restricts its use among young people, via licensing regulations for sellers and the enforcement of age restrictions for consumers, best reduces the threats associated with the plant's use or abuse by adolescents."

While the study found an association between adolescent marijuana use and poor developmental outcomes, it did not prove cause-and-effect.

SOURCES: Edmund Silins, Ph.D., research fellow, faculty of medicine, University of New South Wales, National Drug and Alcohol Research Centre, Sydney, Australia; Richard Mattick, Ph.D., professor of drug and alcohol studies, University of New South Wales, National Drug and Alcohol Research Centre, Sydney, Australia; Paul Armentano, deputy director, NORML, New York City; September 2014, The Lancet Psychiatry